 In the 1600s, most people believed that the Earth was stationary, while the Sun, the Moon and all the other planets orbited around it. Now Galileo, however, was a big proponent of Copernican theory, which said that the Earth moved around the Sun. Now in 1616, Galileo received a letter from a man named Francesco Ingoli, and in this, Ingoli outlined 18 arguments with Galileo about why the Earth could not possibly be moving around the Sun. One of these was that if the Earth really did revolve around the Sun, then it would have to be moving at nearly 30 kilometres every second. So why then, if you jump straight up, doesn't the Earth keep moving, leaving you thousands of metres from where you started when you land? Now Galileo brought a book as a reply to this letter, and in it he considered a metaphor of a ship sailing smoothly at constant speed. For us, it will be a bit easier to think about a train. So let's forget about the motion of the Earth, and just imagine a train moving at 30 kilometres per second, not accelerating and not turning. If you were to stand at the front of this train and jump straight up, why do you land right back where you started, and not all the way at the back of the carriage?